ALIENS: where are you? Our hopes of finding intelligent companionship seem to be constantly receding. Mars and Venus are not the richly populated realms we once guessed at. The icy seas of the outer solar system may hold life, but almost certainly no more than microbes. And the search for radio signals from more distant extraterrestrials has so frustrated some astronomers that they are suggesting we shout out an interstellar “Hello”, in the hope of prodding the dozy creatures into a response.

So maybe we need to think along different lines. Rather than trying to intercept alien communications, perhaps we should go looking for alien artefacts.

There have already been a handful of small-scale searches, but now three teams of astronomers are setting out to scan a much greater volume of space (see diagram). Two groups hope to see the shadows of alien industry in fluctuating starlight. The third, like archaeologists sifting through a midden heap on Earth, is hunting for alien waste.

What they’re after is something rather grander than flint arrowheads or shards of pottery. Something big. Planet-sized power stations. Star-girdling rings or spheres. Computers the size of a solar system. Perhaps even an assembly of hardware so vast it can darken an entire galaxy.

It might seem crazy to even entertain the notion of such stupendous celestial edifices, let alone go and look for them. Yet there is a simple rationale. Unless tool-users are always doomed to destroy themselves, any civilisation out there is likely to be far older and far more advanced than ours.

Humanity has already covered vast areas of Earth’s surface with roads and cities, and begun sending probes to other planets. If we can do all this in a matter of centuries, what could more advanced civilisations do over many thousands or even millions of years?

In 1960, the physicist Freeman Dyson pointed out that if alien civilisations keep growing and expanding, they will inevitably consume ever more energy – and the biggest source of energy in any star system is the star itself. Our total power consumption today is equivalent to about 0.01 per cent of the sunlight falling on Earth, so solar power could easily supply all our needs. If energy demand keeps growing at 1 per cent a year, however, then in 1000 years we’d need more energy than strikes the surface of the planet. Other energy sources, such as nuclear fusion, cannot solve the problem because the waste heat would fry the planet.

In a similar position, alien civilisations could start building solar power plants, factories and even habitats in space. With material mined from asteroids, then planets, and perhaps even the star itself, they could really spread out. Dyson’s conclusion was that after thousands or millions of years, the star might be entirely surrounded by a vast artificial sphere of solar panels.

The scale of a Dyson sphere is almost unimaginable. A sphere with a radius similar to that of Earth’s orbit would have more than a hundred million times the surface area of Earth. Nobody thinks building it would be easy. A single shell is almost certainly out, as it would be under extraordinary stresses and gravitationally unstable. A more plausible option is a swarm: many huge power stations on orbits that do not intersect, effectively surrounding the star. Dyson himself does not like to speculate on the details, or on the likelihood of a sphere being built. “We have no way of judging,” he says. The crucial point is that if any aliens have built Dyson spheres, there is a chance we could spot them.

A sphere would block the sun’s light, making it invisible to our eyes, but the sphere would still emit waste heat in the form of infrared radiation. So, as Carl Sagan pointed out in 1966, if infrared telescopes spot a warm object but nothing shows up at visible wavelengths, it could be a Dyson sphere.

Some natural objects can produce the same effect. Very young and very old stars are often surrounded by dust and gas, which blocks their light and radiates infrared. But the infrared spectrum of these objects should be a giveaway. Silicate minerals in dust produce a distinctive broad peak in the spectrum, and molecules in a warm gas would produce bright or dark spectral lines at specific wavelengths. By contrast, waste heat from a sphere should have a smooth, featureless thermal spectrum. “We would be hoping that the spectrum looks boring,” says Matt Povich at the California State Polytechnic University in Pomona. “The more boring the better.”

Our first good view of the sky at the appropriate wavelengths came when the Infrared Astronomical Satellite surveyed the skies for 10 months in 1983, and a few astronomers have sifted through its data. Vyacheslav Slysh at the Space Research Institute in Moscow made the first attempt in 1985, and Richard Carrigan at Fermilab in Illinois published the latest search in 2009. “I wanted to get into the mode of the British Museum, to go and look for artefacts,” he says.

Carrigan found no persuasive sources, but the range of his search was limited. It would have detected spheres around sunlike stars only within 1000 light years of Earth. This is a very small part of the Milky Way, which is 100,000 light years across.

One reason few have joined Carrigan in the hunt for artefacts is the difficulty of getting funding for such projects. Then last year, the Templeton Foundation – an organisation set up by a billionaire to fund research into the “big questions” – invited proposals for its New Frontiers programme, specifically requesting research that would not normally be funded because of its speculative nature. A few astronomers jumped at the chance to look for alien contraptions and, in October, the programme approved three separate searches. The grants are just a couple of hundred thousand dollars each, but they do not have to fund new telescopes, only new analysis.

One team, led by Jason Wright at Pennsylvania State University in University Park, will look for the waste heat of Dyson spheres by analysing data from two space-based infrared observatories, the Wide-field Infrared Survey Explorer (WISE) and the Spitzer space telescope, launched in 2009 and 2003. Povich, a member of this team, is looking specifically within the Milky Way. Thanks to the data from Spitzer and WISE, Povich should be able to scan a volume of space thousands of times larger than previous searches like Carrigan’s. “For example, if you had a sun-equivalent star, fully enclosed in a Dyson sphere, we should be able to detect it almost anywhere in the galaxy.”

Even such a wide-ranging hunt may not be ambitious enough, according to Wright. He suspects that interstellar travel will prove no harder than constructing a sphere. An alien civilisation with such a high level of technology would spread out and colonise the galaxy in a few million years, building spheres as they go. “I would argue that it’s very hard for a spacefaring civilisation to die out. There are too many lifeboats,” says Wright. “Once you have self-sufficient colonies, you will take over the galaxy – you can’t even try to stop it because you can’t coordinate the actions of the colonies.”

If this had happened in the Milky Way, there should be spheres everywhere. “To find one or a few Dyson spheres in our galaxy would be very strange,” says Wright.

So he is venturing deeper into the cosmos. “A colonised galaxy would quickly go from looking ordinary to very red,” says Wright. “So we are looking for a big bright galaxy in the WISE data that doesn’t have an optical counterpart.” If some clever betentacled ones have conquered and shrouded a galaxy as big as the Milky Way – a Kardashev-III civilisation in alien-hunter jargon – this project could spot their work as much as a billion light years away. And if they have colonised a whole galaxy cluster, even further.

Although it has tremendous reach, the waste-heat approach has its limitations. It won’t flag anything up if ET builds only a thin ring of collectors or a sphere with many gaps, letting a lot of starlight through. That’s where the other two searches come in. They will be looking for relatively small artefacts with the help of the Kepler planet-finding telescope. This spacecraft monitors about 150,000 nearby stars, looking for telltale changes in their brightness, and it has already detected thousands of new planets.

Spot anything unusual

“Now we are looking for oddballs that don’t conform to a natural explanation,” says Andrew Howard at the University of Hawaii in Honolulu. Along with Geoffrey Marcy at the University of California, Berkeley, his team will be searching for unusual patterns of dimming.

Another group, led by Lucianne Walkowicz at Princeton University, will also be searching for anomalies in the data, using a slightly different approach. “Their search relies on having a human being inspect the light curves by eye. Our search uses machine-learning algorithms,” she says.

A light curve is the change in brightness of a star over time, for instance as an object transits across its disc. With an object about the size of a gas-giant planet,the light curves from Kepler can even give an idea of its shape. A Jupiter-sized rectangle would certainly suggest the hand of intelligence.

Such a monolith might also endure longer than a full Dyson sphere – and the longer an artefact lasts, the greater our chances of spotting it. Howard thinks that when bodies circle a star in different planes, their gravity would tend to disturb their orbits, so a Dyson swarm would be unstable and break up if abandoned. A ring or isolated artefact could, however, be stable for billions of years.

All this guesswork seems futile to Walkowicz. “People spend a lot of time trying to psychoanalyse ET, but we have no info about what their technology would be like. The more you try to imagine what aliens would do, the more you limit your scope.” That’s why her team is looking for anything strange. Walkowicz thinks they should turn up interesting candidates in a matter of months.

Those candidates won’t necessarily be limited to “big stuff getting in the way”. The searches could detect anything that modifies starlight. A giant mirror for generating power or driving solar-sail spacecraft would produce distinctive glints. And if ET has the power to tinker with stellar physics – perhaps to prolong their sun’s life or generate useful elements – then artificial variability in the star itself would show up too.

“We know what transits, starspots and flares looks like, so we are looking for any variation we can’t explain through known astrophysics,” says Walkowicz. Of course, just finding something new and strange does not mean aliens are out there. All the researchers would be delighted to discover natural phenomena, too. For some, this is what makes the search worthwhile. “I’m not doing this because I expect to see a Dyson sphere,” says Povich. “That is just an opportunity to sift through a great amount of data, to give us a new perspective on what’s there.”

All of this will satisfy Dyson, an honorary consultant for the New Frontiers programme. “The search for artefacts should not be separate from normal exploring of the universe,” says Dyson. “We explore a huge variety of natural objects, and if something sufficiently weird shows up it might turn out to be artificial.”

Then again, a sufficiently advanced technology might be just too weird for us to see. Even some forms of Dyson sphere would be very hard to spot. The futurist Robert Bradbury suggested that the most efficient set-up for a celestial power station would be a whole series of spheres, nested inside one another like a Russian matryoshka doll, with the cooler outer layers mopping up the waste heat of the inner ones. He envisaged the power being used by a vast computer, so he called this system a matryoshka brain.

The lowest possible temperature of the outer layer of such a set-up would be just above that of the cosmic microwave background radiation at 3 kelvin, making it very difficult to detect from its radiation. Without our knowing it, a matryoshka brain might be lurking nearby, perhaps regarding us with an intellect that is truly vast and cool.

This article appeared in print under the headline “To boldly look”

Stephen Battersby is a consultant for New Scientist based in London

http://www.newscientist.com/article/mg21829112.100-alien-megaprojects-the-hunt-has-begun.html?full=true